a) Field of the Invention
The present invention relates to an eyepiece system which is to be used in telescopes and microscopes or as a magnifying lens for observing magnified images of objects.
b) Description of the Prior Art
Ramsden type eyepiece systems and Kellner type eyepiece systems are known as eyepiece systems which are to be used in telescopes and microscopes, and have simple compositions. Each of these type eyepiece systems is composed of a lens which is disposed on a side near an eye (eye lens) and a field lens is disposed near an image to be magnified.
The Ramsden type eyepiece system is composed of an eye lens which is composed of a total of two lens elements, i.e., an eye lens which is composed of a single lens element and a field lens which is composed of a single lens element, whereas the Kellner type eyepiece system is composed of a total of three lens elements, i.e., an achromatic cemented lens which is adopted as the eye lens for correcting lateral chromatic aberration and a field lens which is composed of a single lens element.
The conventional Ramsden type eyepiece system in which the eye lens is composed of a single homogeneous lens element produces chromatic aberration, curvature of field and spherical aberration in large amounts. When an aspherical surface is used in this eyepiece system, it can correct spherical aberration but cannot correct chromatic aberration and curvature of field. The Kellner type eyepiece system which uses the achromatic cemented lens as the eye lens is capable of favorably correcting chromatic aberration and spherical aberration, but produces curvature of field in an amount too large for correction. Further, the Kellner type eyepiece system has a defect that it requires a high maufacturing cost due to the fact that the eye lens is configured as the achromatic cemented lens.
Furthermore, Japanese Patent Kokai Publication No. Sho 62-32414 discloses an eyepiece system which uses a radial type gradient index lens (radial type GRIN lens) but produces spherical aberration and curvature of field in large amounts. No description is made of chromatic aberration in this publication. As eyepiece systems which use new lens materials, there are known those which use radial type gradient index lenses which have refractive indices varying dependently on distances in radial directions (radial type GRIN lenses) like the eyepiece systems disclosed by Japanese Patents Kokai Publication No. Sho 62-32414 and Kokai Publication No. Hei 6-337347. Out of the eyepiece systems mentioned as the conventional examples, an eyepiece system composed of two radial type GRIN lenses, and another eyepiece system composed of a radial type GRIN lens and a homogeneous lens are known as eyepiece systems which are composed of a small number of lenses.
Furthermore, a lens system described in Applied Optics Vol. 22 pp.on and after 407 is known as an eyepiece system using an axial type GRIN lens whose refractive index varies in a direction along an optical axis (in a direction of thickness of a lens). This literature describes examples in which axial type GRIN lenses are applied to Ramsden type eyepiece systems and Kellner type eyepiece systems.
Moreover, another eyepiece system using a radial type GRIN lens which is disclosed by Japanese Patent Kokai Publication No. Hei 6-337347 has a defect that when it is configured to have favorable optical performance within a wide visual field, it has a long outside diameter and refractive indices which are largely different between an axial portion and a marginal portion thereof, whereby it can hardly be manufactured in practice.
In addition, design examples of eyepiece systems using the axial type GRIN lenses described in the above-mentioned literature exhibit aberration correcting effects which are on the order similar to those of the conventional Ramsden type and Kellner type eyepiece systems using aspherical surfaces, and have a defect that they are incapable of correcting curvature of field and chromatic aberration.